In Australia using first names is the norm. Adults introduce themselves to children by their first name, the only place they call adults Mr. or Mrs. is in school. Outside of school I've only addressed the very elderly that way and only heard it used with the very elderly, such as in a doctor's office. I've wondered if this is more casual than in the US because of past interactions I've had.

Perhaps, it is more true in the South (US), here we address elders as Mr. or Ms. So-and-so. Children of my friends call me Mr. [First Name]. It denotes both familiarity and respect.

I love "Mr/Ms Firstname". Less formal than the way I was raised (further north) to call adults Mr/Ms Lastname, or occasionally Mr/Ms L if I knew them well. But not as informal as just first names. I have to admit, I'm not comfortable when someone I'm not close to tells their, say, three-year-old to call me Firstname. Sometimes I let it go; sometimes I tell them it's Ms. Firstname, please.